The surrounding shores of Plymouth Sound and Catwater were next examined, with a view to deter mine from what quarter materials for this great un dertaking could most conveniently be obtained, as to quality, cheapness, and celerity of conveyance. On the west or Cornish side of the sound, nothing appears but hard granite; at the head of the sound and in Catwater, on the Devonshire side, all is mar ble and limestone. In Catwater alone, it was esti mated, on a rough calculation, that 20 millions of tons might be procured in blocks fit for the work, which was about ten times the quantity that would probably be wanted. The time required for the completion of the work, would depend on a variety of circumstances. It is obvious that, if the two sides of the sound had furnished proper materials for the purpose, the time would considerably have been abridged, as, in that case, when the wind was easter ly, vessels might deposit stones on the eastern end of the Breakwater, and in westerly winds, on the western extremity, and the work would thus be pro ceeding with an uninterrupted progress ; whereas if the stone was to be brought from one point, and that point on the shore of Catwater, a strong southerly and south-westerly wind, those most prevalent in this country in the winter months, would generally im pede and frequently render it impossible for vessels to go off with their cargoes.
Catwater, however, having many advantages, and especially for the convenience of loading the vessels, and the facility of procuring blocks from the quer- . ries of any size, was considered, on the whole, as en titled to the preference over any other place. Be sides the quarries here being in the neighbourhood of villages, lodgings and conveniences would be af forded for the workmen; and, on the whole, it was calculated that the work might be completed from hence at a cheaper rate, and perhaps in less time, than from situations much nearer to it, but •much more exposed to the wind and waves.
An estimate of the expence could not be made with any degree of accuracy, as no correct section of the bottom had been taken. Supposing, however, the great Breakwater to be 1700 yards in length, 80 feet in width at the top, when carried 10 feet above low water of spring-tides, with a slope on the southern 'or sea side, of three horizontal to one perpendicular, and, on the sound or land side of one and a half ho rizontal to one perpendicular, it was calculated that the whole mass of stone required, would be about two millions of tons. If then 100 sail of vessels of 50 tons burthen each were employed in carrying stone, and that each vessel was to carry only 100 tons a week, the quantity deposited in one week, would amount to 10,000 tons, or say 500,000 tons a year; and, at this rate, the Breakwater would be completed in four years; but making allowance for time lost in preparations, contingent delays and un favourable weather, and deductions in the quantity of stone for the shallow parts over which the line of the breakwater was carried, the completion of the , work might safely be calculated within the period of six 'vars.
Nor would the building of the pier from Andurn point, if so determined, increase the time of comple tion. If carried from the shelving rocks within the point, leaving a passage between them, the pier would require about 860,000 tons of stone, which, by em ploying about 80 vessels, might be deposited in three years.
It was recommended by the gentlemen above men ' doped, that the great Breakwater should be begun oa the Shovel and extended on both sides of it, as, by so doing, the effect produced on the sound would be observed as the work proceeded; and that buoys should be placed along the line, so that the whole of the vessels "employed might, if necessary, deposit their cargoes at the same time without interrupting rich other.
The rough estimate for completing this great na tional work, made on the grounds above stated, was as follows: and seriously considered, that Mr Yorke determined to carry into execution this great undertaking. The principal objection started against it was that it might cause the anchorage in the Sound to be de stroyed in the course of time by the deposition of mud and silt along the whole eddy within it. There does not, however, appear to be any solid ground for this objection, The water brought by the tides from the sea is at all times perfectly clear and trans parent, and that which proceeds from Hamosze and supplied by the Tamar and the Tavy, is almost wholly free from any alluvial matter, thew rivers holding their course through a fine granite soil. The fact is suthciendy proved by the circumstance of no deposition taking place in the recesses of Ramo we along the dock-yard wall, which lead into the docks, nor in the numerous eddies that are caused by the projecting jetties and salient angles of that wall. Another objection started against the under taking was. that, by the diminished quantity of water thrown in by the tide into Hamoaze and Catwater, the Sound would gradually till up and these harbours be destroyed. No perceptible alteration, however, has as yet taken place in the height of the eater in Hamoase, or in the strength or set of the tides.
A rock of limestone, or rather gray marble, si tuated at Oreston, on the eastern shore of Cat water, consisting of a surface of 25 acres, was pur chased from the Duke of Bedford for the sum of L.10,000; quays for shipping the stone were erect ed in front of it ; iron railways leading from the quarries to the quays were laid down ; ships were hired by contract to carry off the stone, and others built at the dock-yard. Whidby was appointed to superintend the work. The quarries were opened on the 7th August 1812; the first stone deposited on the lath of the same month; and, on the 31st March 1818, the breakwater made its first appear ance above the surface of the Sound low water of the spring-tide. The system of quarrying the stone! is conducted with admirable skill, and stones of the proper size obtained with less waste of small rubble than might be expected. In working these quarries an extraordinary phenomenon was discovered in the very body of the great mats of this old marble rock. At the depth of 65 feet from the summit of the rock, and 25 from the margin of the sea, a cavity, or ra ther a nodule of clay was discovered, of 25 feet long and 12 square, or thereabouts, in the midst of which were found several bones of the rhinoceros, in a more perfect state, and containing less animal matter in them, than any fossil bones that have yet been dug out of rock or earth.